I feel for you in this stressful time, and I think Jeff and Fran both gave very good advice and information. I know I was always looking for answers when my mother's health took a turn for the worse or the better, and I still don't know if the chemo weakened and killed her a little early, or if the disease was just progressing on its natural, devastating course.
As I always say, this disease is so different in each individual, so there is really no answer. The chemo can be quite debilitating for some people, so he may be feeling the side effects of that, and I think those can last longer than they lead you to believe (Xeloda didn't bother my mother as much as the Gemzar - and I think mouth ulcers are definitely a chemo side effect). On the other hand, there is the possibility that the disease is progressing. Don't beat yourself up thinking that you wasted your time with chemo and made him suffer - it's a decision that's purely individiual and you never know if it may work or not, and I'm sure you are doing the very best for him and he thanks you for it. I often feel guilty that I allowed my mother to endure the pain of two rounds of chemo, but we had to take a shot and we couldn't know that she was too far along and too sensitive to the treatment. If you feel your dad's quality of life is being compromised by the chemo, I would see if he was amenable to taking a break or discontinuing completely - he may be relieved at the suggestion - or he may want to continue, and it is his right to choose either option.
As Fran said, the carers are burdened with the big questions and worries, and we all do the best we can in a sea of uncertainty. And as Fran also said, sometimes people bounce back SEVERAL times when you think all hope is lost. Hopefully the oncologist can give you a better picture of what's going on, but I just wanted to give my two cents of support and say that you're doing a great job and there are just some things you have no control over, much as you try. Jeff's suggestion about hydration is VERY important, too. I wish you and your dad the best of luck.
Fran - so sorry to hear your husband is doing poorly, I know how gut-wrenching it is. I'll just hope that's he's having another slump that he can bounce back from -- but it's hard to live this roller coaster, hard for the patient AND the caregiver.
Much love sent to all of you,